The long-term objective of this research is to elucidate the molecular, cellular, and biochemical mechanisms underlying cell-mediated cytotoxicity in strains of the snail, Biomphalaria glabrata, that are susceptible or resistant to infection with larval stages of the human blood fluke, Schistosoma mansoni. The system of study will be the NlH-Sm-PR-1 strain of S. mansoni and three strains of the snail intermediate host: the schistosome-susceptible M-line, and resistant 1O-R2 and 13-16-R1 strains of B. glabrata. Using this model a comparative functional, biochemical, and molecular approach will be used to determine if the reactive oxygen intermediates (ROIs), superoxide and hydrogen peroxide, are produced by snail hemocytes and if they are involved in snail hemocyte-mediated parasite killing. There are three specific aims to this research. The first aim is to determine the production and role of hemocyte-derived ROIs in larval schistosome killing. Baseline ROI levels will be quantified using an in vitro phagocytosis assay, in the presence or absence of snail plasma and"'or parasite excretory-secretory (E-S) products, while hemocyte ROI production during hemocyte-sporocyst contact will be determined using an in vitro encapsulation assay. ROI-mediated sporocyst killing will be determined using an established live-dead assay. The second aim is to isolate specific snail plasma and parasite E-S products that directly affect ROI production by identifying those products that effect hemocyte ROI production and phagocytosis separately. Plasma and E-S fractions will be obtained using molecular ultrafiltration. Components of fractions will be isolated using high performance liquid chromatography, ion exchange chromatography, and electrophoretic techniques. Effects of these components and fractions on hemocyte ROI production and killing of sporocysts will be determined using in vitro phagocytosis, live-dead, and encapsulation assays. The third aim is to isolate snail interleukin-1 and to further characterize a parasite-derived E-S antioxidant molecule. The function of both molecules will be evaluated for their influences on ROI production and effect during hemocyte encapsulation of the parasite. Peptide fragments from both molecules will be N-terminally sequenced in preparation for the future isolation and sequencing of genes associated with schistosome killing. This research will contribute significantly to the understanding of vector competence in schistosome-snail interactions, which should ultimately lead to novel and more efficient approaches to the control of schistosomes.